Method for improving heat and mass transfers toward and/or through a wall

ABSTRACT

PCT No. PCT/FR92/00933 Sec. 371 Date Jun. 10, 1993 Sec. 102(e) Date Jun. 10, 1993 PCT Filed Oct. 8, 1992 PCT Pub. No. WO93/07433 PCT Pub. Date Apr. 15, 1993The invention relates to a method for improving heat and mass transfers to and/or through a wall, and if need be, to such a method applied to a permeable wall. The invention also concerns a wall and conductive material pair having improved heat and mass transfer characteristics. The method of the invention is useful for all techniques requiring both heat transfer and flow of gaseous phase to or through a wall.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for improving heat and masstransfers towards and/or through a wall and, if appropriate, to such amethod intended to be used with a permeable wall. The present inventionalso relates to a pair consisting of a wall and of a conducting materialhaving improved heat and mass transfer characteristics.

Certain chemical or physicochemical techniques require the transit of agas towards a wall separating the operating space into two regions, inwhich wall the temperature must additionally be controlled, byintroduction or extraction of heat, in order to make it possible or easyto use these techniques which could, for example, involve passing thesaid gas from one region to the other through the wall. The wall canthus define an open or closed region constituting a medium containingone or a number of gaseous, liquid or solid materials, the said mediumbeing stationary or non-stationary.

Thus, the problem is posed of maintaining the temperature of the wall,for example by heating, constant over all its useful surface whileproviding free passage of the gas towards the wall.

There are also problems with heat exchangers in which the exchange iscarried out via an impermeable conducting wall which separates twofluids, one at least of which is in the gaseous state. In effect, inthis type of exchanger, the heat transfer coefficient between thegaseous fluid and the wall, and consequently the other fluid, is verylow.

In current methods or devices, it is possible either to improve the heattransfer towards a wall by depositing, on the latter, a good thermallyconducting material which has the associated disadvantage of preventingthe free circulation of the gas in the vicinity of the wall, or ofpromoting the passage of the gas by freeing the wall of all obstacles,in which case the heat transfer is reduced and additionally very badlycontrolled.

SUMMARY OF THE INVENTION

The subject of the present invention is to overcome these disadvantagesby proposing a method which makes it possible to provide both transit ofthe gas towards the wall and the heat transfer.

The subject of the present invention is a method for improving transfersof heat and of material in gaseous form, in the vicinity of a wallseparating an operating space into two regions, characterised in thatthere is attached to the said wall, on at least one of its faces, aporous solid phase having a high thermal conductivity which carries thegaseous effluent, under the action of a flow, into contact with the walland transmits, to the wall, the heat supplied by a thermal source.

The method of the invention applies as much to the processes involving atransfer of materials from one region to the other through the saidwall, which will then have a certain permeability, as to the processesin which there will simply be a heat transfer between the two regions,the wall then being impermeable.

In an embodiment variant, the method is implemented in an operatingspace separated by a permeable wall into two regions, one of which isthe site of a chemical or physicochemical reaction; at least one of thefaces of the said wall, especially that which is on the side of theregion containing the heat source, is attached to a conducting poroussolid phase which will carry the gaseous effluent to the vicinity of thewall and will transmit the heat arising from the source. This methodwill make it possible to precisely control the temperature of the walland thus to control both the temperature conditions of the chemical orphysicochemical reaction and the amounts of gas to be transferred fromone region to the other of the operating space.

Permeable wall is understood to mean a wall capable of allowingcompounds in the gaseous phase to pass under certain conditions oftemperature and pressure. This property, denoted by the name ofpermeability, is modified by a temperature variation and can, undercertain conditions, become virtually zero.

There will be used, according to the precise application of the method,walls whose permeability will vary in a substantially linear way as afunction of the temperature or walls having sudden permeabilityvariations linked to temperature variations.

The invention also relates to a wall/conducting material pair comprisinga wall to which, in order to improve mass and heat transfer towardsand/or through the wall, a porous solid phase having a high thermalconductivity is attached.

The permeable walls used in the method of the invention are of theorganic or inorganic membrane type used in separation, gaseous fluiddiffusion or catalytic reaction processes.

They could be dense membranes, that is to say membranes providingmaterial transfer by dissolution of the compound to be transferred inthe material of the wall, then diffusion and finally expanding out. Theycould also be porous membranes, that is to say membranes providingtransfer through their pores.

By way of examples of organic membranes which can be used in the methodof the invention, there may be mentioned polymeric compounds such ascellulose compounds, especially acetates, polyacrylonitrile, siliconerubber, polycarbonate-rubber-silicone copolymers,polytetrafluoroethylene, poly(vinyl chloride), polysulphones,polyamides, poly(vinyl acetate)s, polycarbonates, polyphosphazenes, andthe like.

By way of examples of inorganic membranes, there may be mentioned

alumina or ceramics based on alumina, on zirconium oxide or on titaniumoxide,

silica,

silica-based glasses made porous, if appropriate, by an acid-basedtreatment,

sintered metals, for example nickel or a stainless steel,

dense metals such as palladium or its alloys, or silver, and the like.

These membranes could, if appropriate, be deposited on a sinteredsupport providing mechanical strength.

The size of the pores of the porous inorganic membranes is generallybetween 10 and 2000 Å.

In the method of the invention, the wall is attached to a porous solidphase which comprises expanded graphite which is advantageouslyrecompressed to substantially reduce its volume and to improve itsconductivity.

By virtue of its low density, which is between 0.001 and 1.5, theexpanded graphite remains extremely porous and thus makes possible thefree passage of a gas while providing good thermal conductivity linkedto the nature of the graphite.

According to a specific aspect of the invention, expanded graphite isused, recompressed so that it has anisotropic heat transfer properties,which will be particularly advantageous for efficiently controlling thetemperature level of the wall. Its density will then be between 0.02 and1.5. Its thermal conductivity will generally be between 0.5 and 30W/m/K.

The recompressed expanded graphite has a coefficient of anisotropygenerally of between 5 and 150. This coefficient is determined by theratio of the thermal conductivity of the graphite measured along adirection D1 to the thermal conductivity of the graphite measured alonga direction D2 perpendicular to the direction D1.

The amount of expanded graphite used will depend mainly on the distancefrom the heat source whose transfer it has to provide.

The present invention also finds its use in improving heat exchangesbetween two fluids, one at least of which is in the gaseous state,separated by an impermeable conducting metal wall. In this type ofapplication, two fluids having different thermal levels are caused tocome into contact with the opposite faces of a wall in order to exchangeheat there. The coefficient of transfer between a turbulent gas and ametal wall is low, usually between 10 and 40 W/m² /°C. and ranging, incertain cases, up to 80 W/m² /°C.

In another variant of the method, the subject of the present inventionis to improve heat transfer between a fluid and a conducting metal wallby attaching, to at least one of the faces of the wall on the side ofthe gaseous fluid, a porous solid phase having a high thermalconductivity. With such a system, the heat transfer coefficient canreach 300 W/m² /°C.

The porous solid will make it possible to considerably increase theactive surface for exchange between the gaseous fluid and the metal wallwhere transfer takes place.

In the case where the exchange relates to two gaseous fluids, it will beadvantageous to deposit the conducting porous solid on each face of thewall.

The walls used in this variant of the method are metal walls which areconventional in this type of exchanger.

The conducting porous solid used will advantageously be the recompressedexpanded graphite described above, a degree of recompression beingchosen which produces a solid having a porosity sufficient to providethe gaseous effluent with a pressure drop compatible with this type ofthermal exchange technique.

Other characteristics and advantages of the present invention willbecome more clearly apparent on reading the description of the examplesof devices and methods below, made with reference to the appendeddrawings:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an assembly of wall/porous solidphase pairs which can be used for the implementation of the method ofthe invention;

FIG. 2 is a schematic sectional view of a pair consisting of a wall andof a conducting material according to another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As represented in FIG. 1, pipes 10, six in the example illustrated, ofcircular cross-section, defined by walls 17 of membrane type, arearranged in an orderly way in a porous solid 12 which, in the exampleillustrated, is of recompressed expanded graphite. A fluid passes insidepipes 10 either in a parallel fashion or, in the example illustrated, inalternate directions as represented by the arrows 14. Heat emanatingfrom a source which is not shown, arrives in the expanded graphite asrepresented schematically by arrows 16. Gas, arising from a source whichis not shown, moves inside the porous graphite towards the walls 17 ofthe pipes 10 as shown schematically by the arrows 18 and, by virtue of apressure difference between the phase 12 and the fluid phase present inthe pipes 10, passes through the wall of the tubes. The amount of gaspassing through the wall depends, under identical pressure conditions,on the temperature applied to the wall via heat conveyed according tothe arrows 16.

The assembly in FIG. 1 finds its use in processes where the aim is tointroduce a gas into a space situated beyond a wall in order to make itreact, whether chemically or physically, with a fluid moving inside thepipes and where the temperature level of the wall has an influence onthe implementation of the process.

EXAMPLE 1

By way of example of a method capable of using the subject of theinvention, there may be mentioned partial oxidation reactions ofhydrocarbons. In effect, these reactions have three maincharacteristics:

their high exothermicity, which can bring about reaction runaway,

yields always reduced by complete oxidation reactions,

essential control of the amounts of oxygen present in the medium inorder to minimise side reactions.

This is the reason why it has already been proposed to use membranemethods which enable the oxygen required for the reaction to beintroduced along the whole length of the reactor.

The use, in this type of method, of the membrane wall/conducting poroussolid pair according to the invention makes it possible, on the onehand, to extract the heat of the reaction and, on the other hand, tocontrol the injection of oxygen by regulating the temperature of thewall.

EXAMPLE 2

The method of the invention can be used in a wide range of chemicalmethods which involve a mass transfer in the gaseous state and a controlof the temperature of the medium by heat transfer.

The method for dehydrogenation of ethylbenzene to styrene is anotherexample of the use of the method according to the invention.

The dehydrogenation reaction of ethylbenzene to styrene is anendothermic catalytic reaction. To increase the conversion, it isadvantageous, on the one hand, to increase the temperature and, on theother hand, to draw off the hydrogen produced as it is formed. Thisextraction can be carried out using an inorganic membrane whose poresize is adjusted to allow hydrogen to pass selectively.

If, in the process used, no heat is introduced, the temperature of thereaction mixture falls very quickly. This decrease thus limits theoverall conversion. By attaching the conducting porous solid to themembrane wall, it will be possible to make the membrane reactorisothermal. Simultaneously, the rate of transfer of hydrogen through themembrane will be controlled to the extent that the flow is proportionalto <T, T being the temperature of the membrane.

EXAMPLE 3

The example below illustrates the advantage of regulating the walltemperature in a process for separation by membrane.

In a conventional device consisting of two chambers separated by amicroporous inorganic membrane made of alumina, the pores of which, ofthe order of 1000 Å, are filled with n-hexadecane, a mixture of thefollowing composition by volume is treated under nominal operatingconditions (20° C.):

Inerts: 2.7%; CH₄ : 90.8%; C₂ H₆ : 4.9%;

C₃ H₈ : 1.19%; i-C₄ H₁₀ : 0.26%; n-C₄ H₁₀ : 0.15%

Under these nominal conditions, the permeability (p) and the selectivitywith respect to methane obtained for the components of the mixtures withthe membrane used are specified in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        components  p(m.sup.3 · m/m.sup.2 · s ·                                       selectivity                                        ______________________________________                                        CH.sub.4    0.24           1                                                  C.sub.2 H.sub.6                                                                           0.9            3.8                                                C.sub.3 H.sub.8                                                                           2.7            11.3                                               i-C.sub.4 H.sub.10                                                                        5.0            20.6                                               n-C.sub.4 H.sub.10                                                                        9.0            37.5                                               ______________________________________                                    

The same operation is carried out using the same device, the injectionchamber of the mixture being equipped with a layer of recompressedexpanded graphite of density [lacuna] attached to the wall and connectedto a heat source which makes it possible to provide heating of the wallin order to raise the temperature to 100° C.

The results obtained under these conditions are shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        components    permeability                                                                             selectivity                                          ______________________________________                                        CH.sub.4      0.49       1                                                    C.sub.2 H.sub.6                                                                             1.14       2.32                                                 C.sub.3 H.sub.9                                                                             2.28       4.65                                                 i-C.sub.4 H.sub.10                                                                          3.34       6.81                                                 n-C.sub.4 H.sub.10                                                                          5.34       10.9                                                 ______________________________________                                    

It can be observed that under these conditions the permeability of thelight gases is increased (doubly so for methane) to the detriment of theheavy compounds.

Adjustment of the temperature of the wall makes it possible to controlthe permeability, and thus the flow, and the selectivity.

EXAMPLE 4

There is represented, in FIG. 2, a heat exchanger 20 formed from animpermeable wall 22 which, in the example illustrated, is planar,separating two fluid phases 24 and 26 of which at least one 24 is in thegaseous state. A porous solid phase 28 having a high thermalconductivity is attached to at least the face 23 of the wall 22 on theside of the gaseous fluid 24. In the example illustrated, where the twofluids 24 and 26 are in the gaseous state, the porous solid phase 28 isattached to both sides of the wall 22 on the faces 23 and 25. The twogaseous fluids 24 and 26 are caused to move in the porous solid phase 28as shown schematically by the arrows 30 and 32.

The porous solid phase 28 comprises recompressed expanded graphitehaving a density between 0.02 and 1.5. Preferably, the recompressedexpanded graphite has, as a result of its recompression, anisotropicthermal conductivity characteristics. Thus, the thermal conductivity inthe direction D, normal to the surface of the wall 22, is markedly moresignificant than that in a direction D2 parallel to the wall.

The heat transfer coefficient of the gaseous fluid 24, 26 to the wall isbrought, by virtue of the invention, to a value between 200 and 300 W/m²/°C. In order to achieve an optimum transfer coefficient, the density ofthe anisotropic recompressed expanded graphite is of the order of 0.2 to0.4 with a porosity of 0.9 to 0.82.

In certain types of heat exchangers, the wall 22 can have a tubularshape.

The method of the invention can naturally be used in any physicochemicaltechnique requiring both heat transfer and free circulation of a gaseousphase towards and/or through a wall.

In addition to the examples described above, there may be mentioned, ina non-limiting way, the practical applications below:

Aeration or oxygenation of a liquid or a gas.

Halogenation reaction of hydrocarbon compounds.

Gas separation through a porous or semi-permeable membrane/wall, ormembrane/wall of permeability according to temperature: wall havingselective permeability; filters.

Devices intended for distributing/releasing a gas through a porous wallat a given temperature, either for producing a mixture or for storingthe gas in an enclosure within which it is trapped by temperaturevariation.

We claim:
 1. Method for improving transfers of heat and of material ingaseous form, in the vicinity of a permeable wall separating anoperating space into two regions, comprising attaching to said permeablewall a porous solid phase which carries a gaseous effluent, under theaction of a flow, into contact with the wall and transmits to andthrough the wall heat supplied by a thermal source, said porous solidphase being attached at least to the face of the permeable wall directedtowards said thermal source and consisting of at least partiallyrecompressed expanded graphite having a density between 0.001 and 1.5and a thermal conductivity between 0.5 and 20 w/m/K, whereby there iscontrolled passage of the gaseous effluent from one region to the other.2. Method according to claim 1, wherein the wall is an organic membrane.3. Method according to claim 1, wherein the wall is an inorganicmembrane.
 4. Method according to claim 1, wherein the graphite hasanisotropic heat transfer characteristics.
 5. Wall/conducting materialpair comprising a permeable wall to which a porous solid phase andconsisting of at least partially recompressed expanded graphite having adensity between 0.001 and 1.5 and a thermal conductivity between 0.5 and20 w/m/K is attached in order to improve mass and heat transfer of agaseous effluent towards and through the permeable wall.
 6. Pairaccording to claim 5, wherein the wall is tubular.
 7. Pair according toclaim 5, wherein wall comprises an organic membrane.
 8. Pair accordingto claim 5, wherein the wall comprises an inorganic membrane.
 9. Pairaccording to claim 8, wherein the wall contains pores whose size isbetween 10 and 2000 Å.
 10. Pair according to claim 5, wherein the poroussolid phase comprises expanded graphite having a density between 0.001and 0.02.
 11. Pair according to claim 5, wherein the expanded graphitehas anisotropic heat transfer characteristics.